The light transmitting performance of an optical fiber is highly dependent upon the properties of the polymer coating that is applied to the fiber during manufacturing. Typically a dual-layer coating system is used where a soft inner-primary coating is in contact with the glass fiber and a harder, outer-primary or secondary coating surrounds the inner-primary coating. The hard coating allows the fiber to be handled and further processed, while the soft coating plays a key role in dissipating external forces and preventing them from being transferred to the fiber where they can cause microbend induced light attenuation.
The functional requirements of the inner-primary coating place various requirements on the materials that are used for these coatings. The Young's modulus of the inner-primary coating is generally less than 1 MPa, or less than 0.5 MPa. The glass transition temperature of the inner-primary coating is less than 5° C., and is ideally about −20° C. or less to ensure that the coating remains soft when the fiber is subjected to low temperatures. In order to ensure uniform deposition on the fiber, the coating is applied to the fiber in liquid form and must quickly form a solid having sufficient integrity to support application of the outer-primary coating. Also, the tensile strength of the coating, which generally decreases as the modulus decreases, must be high enough to prevent tearing defects during draw processing or subsequent processing of the coated fiber during cabling, etc. The functional requirements for the outer-secondary coating include properties that provide mechanical rigidity. The Young's modulus of the outer-secondary coating is generally greater than 1200 MPa, or greater than 1400 MPa. The glass transition temperature of the outer-secondary coating is preferably greater than 30° C., or greater than 40° C., or greater than 50° C.
To meet these requirements, optical fiber coatings have traditionally been formulated as mixtures of radiation-curable urethane acrylate oligomers and radiation-curable acrylate functional diluent monomers. Upon exposure to light and in the presence of a photoinitiator, the acrylate groups rapidly polymerize to form a crosslinked polymer network which is further strengthened by the hydrogen bonding interactions between urethane groups along the oligomer backbone. By varying the chemical formula, molecular weight, and relative proportions of acrylate monomer(s) and urethane acrylate oligomer(s), it is possible to form coatings having very low modulus values and low glass transition temperatures while still having sufficient tensile strength to function as a primary coating as well as coatings having sufficiently high modulus values and glass transition temperatures to function as secondary coatings. Numerous optical fiber coating formulations have been disclosed in the art in which the composition of the coating formulation has been varied to achieve different property targets in cured coatings that cover a wide range of characteristics.
Diluent monomers generally have lower molecular weights than the oligomer components of coating compositions and are included in part to facilitate processing by controlling the viscosity of the coating composition. Diluent monomers are also typically reactive and participate in the curing reaction that produces the ultimate coating for the fiber. Diluent monomers are often monofunctional acrylate monomers. A common class of diluent monomers is the alkoxylated alkyl acrylates, which can be prepared by a reaction of a polyol and an alkyl-substituted methacrylic acid. Under typical conditions, the reaction to form alkoxylated alkyl acrylates is incomplete and the alkoxylated alkyl acrylate product includes residual amounts of unreacted starting materials. Since purification of the alkoxylated alkyl acrylate product is expensive and since the presence of residual unreacted starting materials in coatings formed by curing compositions containing alkoxylated alkyl acrylate monomers does not significantly affect the mechanical and tensile properties of the coatings, alkoxylated alkyl acrylate monomers are often used in an unpurified form and coating compositions based on alkoxylated alkyl acrylate monomers may include residual unreacted starting materials used in the synthesis of alkoxylated alkyl acrylate monomers.
Although the presence of unreacted starting materials from the synthesis of alkoxylated alkyl acrylate monomers in the coating composition does not significantly impact the curing process or the properties of coatings cured from the coating composition, the unreacted starting materials may become incorporated in fiber coatings. It has recently been recognized that impurities and residual matter present in fiber coatings can leach out of the coating as the fiber ages. Concerns arise if components that leach from fiber coatings are toxic or harmful to the environment.
In coatings that contain alkoxylated alkyl acrylate monomers, recent attention has focused on the leaching of residual unreacted hydroxyl compounds, such as ethoxylated nonylphenol, from fiber coatings. Ethoxylated nonylphenol and other alkoxylated alcohols may be referred to herein as polyols. The European Union, for example, has recently identified ethoxylated nonylphenol as a substance of concern. Ethoxylated nonylphenol is a staring material for ethoxylated nonylphenol acrylates, which are widely used as diluent monomers in fiber coatings. Standard commercial ethoxylated nonylphenol acrylate monomers typically include 1 wt % or more of unreacted ethoxylated nonylphenol. The residual amounts of unreacted ethoxylated nonylphenol are present in fiber coatings formed from ethoxylated nonylphenol acrylate monomers and are susceptible to leaching from the fiber coating. When leached, ethoxylated nonylphenol can decompose to form nonylphenol. From an environmental standpoint, nonylphenol is a regulated substance because it is known to be hazardous to marine life. Current European Union regulations specify a maximum level of 0.1 wt % of ethoxylated nonylphenol.
Given the advantageous properties of coatings formed from compositions that include alkoxylated alkyl acrylate monomers, it would be desirable to develop coating compositions based on alkoxylated alkyl acrylate monomers that contain reduced levels of unreacted ethoxylated nonylphenol.